[PATCH v2] xen: xen-pciback: Remove create_workqueue

[Bhaktipriya Shridhar]

System workqueues have been able to handle high level of concurrency
for a long time now and there's no reason to use dedicated workqueues
just to gain concurrency.  Replace dedicated xen_pcibk_wq with the
use of system_wq.

Unlike a dedicated per-cpu workqueue created with create_workqueue(),
system_wq allows multiple work items to overlap executions even on
the same CPU; however, a per-cpu workqueue doesn't have any CPU
locality or global ordering guarantees unless the target CPU is
explicitly specified and thus the increase of local concurrency shouldn't
make any difference.

Since the work items could be pending, flush_work() has been used in
xen_pcibk_disconnect(). xen_pcibk_xenbus_remove() calls free_pdev()
which in turn calls xen_pcibk_disconnect() for every pdev to ensure that
there is no pending task while disconnecting the driver.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@xxxxxxxxx>
---
 Changes in v2:
	-Changed cancel_work_sync to flush_work
	-Changed commit description

 Feedback to update a comment was received in v1 from David Vrabel. It has not
 be included in v2 since some clarification was required. Will include it in
 v3 once the details about the content and the placement of the comment are
 received.

 drivers/xen/xen-pciback/pciback.h     |  1 -
 drivers/xen/xen-pciback/pciback_ops.c |  2 +-
 drivers/xen/xen-pciback/xenbus.c      | 10 +---------
 3 files changed, 2 insertions(+), 11 deletions(-)

diff --git a/drivers/xen/xen-pciback/pciback.h b/drivers/xen/xen-pciback/pciback.h
index 4d529f3..7af369b6 100644
--- a/drivers/xen/xen-pciback/pciback.h
+++ b/drivers/xen/xen-pciback/pciback.h
@@ -55,7 +55,6 @@ struct xen_pcibk_dev_data {

 /* Used by XenBus and xen_pcibk_ops.c */
 extern wait_queue_head_t xen_pcibk_aer_wait_queue;
-extern struct workqueue_struct *xen_pcibk_wq;
 /* Used by pcistub.c and conf_space_quirks.c */
 extern struct list_head xen_pcibk_quirks;

diff --git a/drivers/xen/xen-pciback/pciback_ops.c b/drivers/xen/xen-pciback/pciback_ops.c
index 2f19dd7..f8c7775 100644
--- a/drivers/xen/xen-pciback/pciback_ops.c
+++ b/drivers/xen/xen-pciback/pciback_ops.c
@@ -310,7 +310,7 @@ void xen_pcibk_test_and_schedule_op(struct xen_pcibk_device *pdev)
 	 * already processing a request */
 	if (test_bit(_XEN_PCIF_active, (unsigned long *)&pdev->sh_info->flags)
 	    && !test_and_set_bit(_PDEVF_op_active, &pdev->flags)) {
-		queue_work(xen_pcibk_wq, &pdev->op_work);
+		schedule_work(&pdev->op_work);
 	}
 	/*_XEN_PCIB_active should have been cleared by pcifront. And also make
 	sure xen_pcibk is waiting for ack by checking _PCIB_op_pending*/
diff --git a/drivers/xen/xen-pciback/xenbus.c b/drivers/xen/xen-pciback/xenbus.c
index c252eb3..5ce878c 100644
--- a/drivers/xen/xen-pciback/xenbus.c
+++ b/drivers/xen/xen-pciback/xenbus.c
@@ -17,7 +17,6 @@
 #include "pciback.h"

 #define INVALID_EVTCHN_IRQ  (-1)
-struct workqueue_struct *xen_pcibk_wq;

 static bool __read_mostly passthrough;
 module_param(passthrough, bool, S_IRUGO);
@@ -76,8 +75,7 @@ static void xen_pcibk_disconnect(struct xen_pcibk_device *pdev)
 	/* If the driver domain started an op, make sure we complete it
 	 * before releasing the shared memory */

-	/* Note, the workqueue does not use spinlocks at all.*/
-	flush_workqueue(xen_pcibk_wq);
+	flush_work(&pdev->op_work);

 	if (pdev->sh_info != NULL) {
 		xenbus_unmap_ring_vfree(pdev->xdev, pdev->sh_info);
@@ -733,11 +731,6 @@ const struct xen_pcibk_backend *__read_mostly xen_pcibk_backend;

 int __init xen_pcibk_xenbus_register(void)
 {
-	xen_pcibk_wq = create_workqueue("xen_pciback_workqueue");
-	if (!xen_pcibk_wq) {
-		pr_err("%s: create xen_pciback_workqueue failed\n", __func__);
-		return -EFAULT;
-	}
 	xen_pcibk_backend = &xen_pcibk_vpci_backend;
 	if (passthrough)
 		xen_pcibk_backend = &xen_pcibk_passthrough_backend;
@@ -747,6 +740,5 @@ int __init xen_pcibk_xenbus_register(void)

 void __exit xen_pcibk_xenbus_unregister(void)
 {
-	destroy_workqueue(xen_pcibk_wq);
 	xenbus_unregister_driver(&xen_pcibk_driver);
 }
--
2.1.4